RECORD: Darwin, C. R. 1863. On the existence of two forms, and on their reciprocal sexual relation, in several species of the genus Linum. [Read 5 February] Journal of the Proceedings of the Linnean Society (Botany) 7: 69-83. 1 text figure.

On the existence of two forms, and on their reciprocal sexual
relation, in several species of the genus Linum.1 By CHARLES DARWIN, M.A., F.R.S., F.L.S., &c.

[Read February 5, 1863.]

THE crimson Linum grandiflorum presents two forms,
occurring in about equal numbers, which differ little in structure, but
greatly in function. The foliage, corolla, stamens, and pollen
(examined

1 The genus includes Linseed, known in America as flax. A modified version of this article was reprinted in Forms of Flowers, pp. 81-101. See Correspondence vol. 10. Extracts from this paper, with commentary by Asa Gray, were published in the American Journal of Science and Arts (September 1863): 279-84, see Correspondence vol. 11.

dry, and distended with water) are alike in both forms. The
difference is confined to the pistil: in the one form, which I will
call "short-styled," the column formed by the united styles, and the
short stigmas, together is about half the length of the whole pistil in
the other and "long-styled" form. A more important distinction is, that
the five stigmas in the short-styled form diverge greatly from each
other and pass out between the filaments of the stamens, and thus lie
within the tube of the corolla. In the long-styled form the elongated
stigmas stand nearly upright, and alternate with the anthers. In this
latter form the length of the stigmas varies considerably, their upper
extremities projecting even a little above the anthers, or reaching up
only to about their middle. Nevertheless there is never the slightest
difficulty in distinguishing between the two forms; for, besides the
difference in divergence, the stigmas of the short-styled form never
reach even to the bases of the anthers. In the short-styled, the
papillæ on the stigmatic surfaces are shorter, darker-coloured, and
more crowded together than in the long-styled form: but these
differences seem due merely to the shortening of the stigma; for in the
varieties of the long-styled form with shorter stigmas, the papillæ
are more crowded and darker-coloured than in those with the longer
stigmas. Considering the slight and variable differences between the
two forms of this Linum, it is not surprising that they have
been hitherto overlooked.

In 1861 I had eleven plants growing in my garden, eight of which
were long-styled, and only three short-styled. Two very fine
long-styled plants grew in a bed a hundred yards off, and separated
from the others by a screen of evergreens. I marked twelve flowers, and
put on their stigmas a little pollen from the short-styled plants. The
pollen of the two forms is, as stated, identical in appearance; the
stigmas of the long-styled flowers were already thickly covered with
their own pollen—so thickly that I could not find one bare stigma; and
it was late in the season, namely, September 15th. Altogether, to
expect any result from this trial seemed almost childish. From my
experiments, however, on Primula, which have been laid before
this Society ('Journal,' vol. vi. p. 77),1 I had faith, and did not
hesitate to make the trial, but certainly I did not anticipate the full
result. The germens of these twelve flowers all swelled, and ultimately
six fine capsules (the seed of which germinated this year) and two poor
capsules were produced; only four capsules shanked off. These

two plants produced, before and after and at the time of the trial,
a vast number of flowers, but the germens of not even one swelled. All
these flowers, though their stigmas were so densely covered
with their own pollen, were absolutely barren.

The nine other plants, six long-styled and three short-styled, grew
in the beds of the same flower-garden. Four of the long-styled produced
no seed-capsules; one produced two; but the remaining long-styled plant
grew so close to a short-styled plant that their branches touched, and
this produced twelve capsules, but they were poor. The case was
different with the short-styled plants. The plant which grew in
juxtaposition with the long-styled plant produced ninety-four
imperfectly fertilized capsules containing a multitude of bad seeds,
with a moderate number of good seeds. The two other short-styled plants
grew in a single clump, and were very small, being partly smothered by
other plants; they did not stand very close to any long-styled plants,
yet they yielded together nineteen capsules. These facts seem to show
that the short-styled plants are far more fertile with their own pollen
than the long-styled. We shall immediately see that this is the case in
a slight degree. But I suspect that in this instance the difference in
fertility between the two forms was in part due to a distinct cause. I
repeatedly watched the flowers, and only once saw a humble-bee
momentarily alight on one, and then fly away, as if it were not to its
taste. If bees had visited the several plants, there cannot be a doubt
that the four long-styled plants which did not produce a single capsule
would have borne an abundance. But several times I saw small diptera
sucking the flowers; and these insects, though not visiting the flowers
with anything like the regularity of bees, would carry a little pollen
from one form to the other, especially when growing close together; and
the stigmas of the short-styled plants, diverging within the tube of
the corolla, would be more likely than the upright stigmas of the
long-styled to receive a small quantity of pollen when brought by small
insects. From the much greater number of long-styled than of
short-styled flowers in the garden, evidently the short-styled would be
more likely to receive some pollen from the long-styled, than the
long-styled from the short-styled.

In 1862 I raised thirty-four plants of this Linum in a
hotbed; and these consisted of seventeen long-styled and seventeen
short-styled forms. Seed sown later in the flower-garden yielded
seventeen long-styled and twelve short-styled forms. These facts
justify

the statement that the two forms are produced in about equal
numbers. The first thirty-four plants were kept under a net which
excluded insects. I fertilized heteromorphically fourteen long-styled
flowers with pollen from the short-styled, and got eleven fine
seed-capsules; these contained on an average 8.6 seeds per capsule, but
only 5.6 were apparently good. It may be well to state that ten seeds
is the maximum possible production for a capsule, and that our climate
cannot be very favourable to this North-African plant. On three
occasions I fertilized homomorphically1 the stigmas of altogether nearly
a hundred flowers (but did not separately mark them) with their own
pollen, but taken from separate plants, so as to prevent any possible
ill effects from close interbreeding; and many other flowers were
produced, which, as before stated, would get plenty of their own
individual pollen; yet from all these flowers, borne by the seventeen
long-styled plants, only three capsules were produced; one of these
included no seed, and the other two together gave only five good seeds.
Nor do I feel at all sure that this miserable product of the two
half-fertile capsules from the seventeen plants, each of which must
have produced at least fifty or sixty flowers, is really the result of
their fertilization by their own pollen; for I made a great mistake in
keeping the two forms under the same net, with their branches often
interlocking, and it is surprising that a greater number of flowers
were not accidentally fertilized.

Of the short-styled flowers I fertilized heteromorphically twelve
with the pollen of the long-styled (and to make sure of the result I
previously castrated the majority), and obtained seven fine
seed-capsules. These included an average of 7.6 seeds, but of
apparently good seed only 4.3 per capsule. At three separate times I
fertilized homomorphically nearly a hundred flowers with their own-form
pollen, taken from separate plants; and numerous other flowers were
produced, many of which must have received their own pollen. From all
these flowers borne on the seventeen plants, only fifteen capsules were
produced, of which only eleven contained any good seed, on an average
4.2 per capsule. As remarked in the case of the long-styled plants,
some even of these capsules were perhaps the product of a little pollen
accidentally fallen from the flowers of the other form. Nevertheless
the short-styled plants seem to be slightly more fertile with their own
pollen, in the proportion of fifteen capsules to three, than the
long-styled: the real proportional excess in fertility is probably a
little greater, as the short-styled flowers, when not disturbed, do

1 'when the long- and the short-styled stigmas are fertilized by their own-form pollen the union is said to be "homomorphic"; when the long-styled and short-styled stigmas are fertilized by the pollen of the other form, the union is "heteromorphic."' Darwin 1862, p. 87.

not so surely receive their own pollen as do the long-styled. The
greater self-fertility of the short-styled flowers was, as we have
seen, also shown by the plants left to themselves, and but sparingly
visited by insects, in the flower-garden in 1861, and likewise by those
raised in 1862.

The absolute sterility (judging from the experiments of 1861, and
which is hardly contradicted by those of 1862) of the long-styled
plants with their own-form pollen led me to examine into its apparent
cause; and the result is so curious that it will be worth while to give
most of the experiments in detail. These experiments were tried on
fresh plants, grown in pots and brought successively into the house.

First. I placed pollen from a short-styled flower on the
five stigmas of a long-styled plant, and after thirty hours found them
deeply penetrated by a multitude of pollen-tubes, far too numerous to
be counted; the stigmas had become discoloured and twisted. I repeated
this experiment on another flower, and in 18 hours found the stigmas
penetrated by a multitude of long pollen-tubes. All this is what might
have been expected, as this is a fertile or heteromorphic union. I
likewise tried the converse experiment, and placed pollen from a
long-styled flower on the stigmas of a short-styled flower, and in 24
hours found the stigmas discoloured, twisted, and penetrated by
numerous pollen-tubes; and this, again, is what might have been
expected, as this is a fertile or heteromorphic union.

Secondly. I placed pollen of a long-styled flower on all
five stigmas of a long-styled flower on a separate plant: after 19
hours I rigorously dissected the stigmas, and found only a single
pollen-grain which had emitted a very short tube. To make sure that the
pollen was good, I took in this case, and in most other cases, pollen
either from actually the same anther or from the same flower, and
proved it to be good by placing it on the stigma of a short-styled
plant, and seeing numerous pollen-tubes emitted.

Thirdly. Repeated last experiment, and placed own-form
pollen on all five stigmas of a long-styled flower; and, after 19½
hours, not one single grain had emitted its tube.

Fourthly. Repeated the experiment, with the same result
after 24 hours.

Fifthly. Repeated last experiment, and, after leaving
pollen on for 19 hours, put an additional quantity of own-form pollen
on all five stigmas. After an interval of exactly three whole days, I
rigorously examined the stigmas, which, instead of being dis-

coloured and twisted, were straight and fresh-coloured; and only one
grain had emitted quite a short tube, which could be drawn out of the
stigmatic tissue without being ruptured.

The following experiments are
more striking:—

Sixthly. I placed own-form pollen on three of the stigmas
of a long-styled flower, and pollen from a short-styled flower on the
other two stigmas. After 22 hours these two stigmas were discoloured,
and slightly twisted, and penetrated by the tubes of numerous
pollen-grains: the other three stigmas, covered with their own-form
pollen, were fresh, and all the pollen-grains were loose; but I did not
dissect the whole stigma rigorously.

Seventhly. Experiment repeated in the same manner, with
the
same result.

Eighthly. Experiment repeated, but the stigmas were
carefully examined after an interval of only 5½ hours. The two
stigmas with pollen from a short-styled flower were penetrated by
innumerable tubes; but these were as yet short, and the stigmas
themselves were not at all discoloured. The three stigmas covered with
their own-form pollen were not penetrated by a single pollen-tube.

Ninthly. Put pollen of short-styled on one stigma, and
own-form pollen on the other four stigmas; after 24 hours, found the
one stigma somewhat discoloured, and twisted, and penetrated by many
long tubes: the other four stigmas were quite straight and fresh; but
on dissecting their whole lengths I found that three pollen-grains had
protruded quite short tubes into the tissue.

Tenthly. Repeated the experiment, with the same result
after 24 hours, excepting that only two own-form grains had penetrated
the stigmatic tissue with their tubes, to a very short depth: the one
stigma, which was deeply penetrated by a multitude of tubes from the
short-styled pollen, presented a conspicuous difference in comparison
with the other four straight and bright pink stigmas, in being much
curled, half-shrivelled, and discoloured.

I could add a few other experiments; but those now given amply
suffice to show that the pollen-grains of a short-styled flower placed
on the stigmas of a long-styled flower emit a multitude of tubes after
an interval of from five to six hours, and penetrate the tissue
ultimately to a great depth, and that after twenty-four hours the
stigmas thus penetrated change colour, become twisted, and appear
half-withered. On the other hand, the pollen-grains of the long-styled
flowers placed on their own stigmas, after an interval of a day, or
even three days, do not emit their tubes, or at most only three or four
grains out of a multitude emit their tubes; and these

apparently never penetrate the stigmatic tissue deeply, and the
stigmas themselves do not become discoloured and twisted.

This seems
to
me a remarkable physiological fact. The pollen-grains of the two forms
are undistinguishable under the microscope; the stigmas differ only in
length, degree of divergence, and in the size, shade of colour, and
approximation of their papillæ, these latter differences being
variable and apparently simply due to the elongation of the stigma. Yet
we plainly see that the two pollens and the two stigmas are widely
dissimilar in action—the stigmas of each form being almost powerless on
their own pollen, but causing, through some mysterious influence, by
simple contact (for I could detect no viscid secretion), the
pollen-grains of the opposite form to protrude their tubes. It may be
said that the two pollens and the two stigmas by some means mutually
recognize each other. Taking fertility as the criterion of
distinctness, it is no exaggeration to say that the pollen of the
long-styled Linum grandiflorum (and conversely of the other
form) has been differentiated, with respect to the stigmas of all the
flowers of the same form, to a degree corresponding with that of
distinct species of the same genus, or even of species of distinct
genera.

Linum perenne.—The dimorphism is here more conspicuous,
and has been noticed by several authors. In the long-styled form the
pistil is nearly twice as long as in short-styled; in the latter the
stigmas are smaller and, diverging more, pass out between the filaments
of the stamens. I could detect no difference in the size of the
stigmatic papillæ; in the long-styled form alone the stigmatic
surfaces turn round so as to face the circumference of the flower: but
to this point we shall presently return. Differently from what occurs
in L. grandiflorum, the long-styled flowers have stamens
hardly more than half the length of those of the short-styled. The size
of the pollen-grains is rather variable; after some doubt, I have come
to the conclusion that there is no uniform difference between the
pollen of the two forms. The long stamens in the short-styled form
project to some height above the corolla, and, apparently from exposure
to the light, the filaments are coloured blue. These longer stamens
correspond in height with the lower part of the stigmas of the
long-styled flowers; and the shorter stamens of the latter form
correspond in the same manner in height with the shorter stigmas of the
short-styled flowers.

I raised from seed twenty-six plants, which proved to be twelve
long-styled and fourteen short-styled. They flowered well, but were not
large plants. As I did not expect them to flower so

soon, I did not transplant them, and they unfortunately grew with
their branches closely interlocked. All the plants were covered by a
net, excepting one of each form. First, of the long-styled flowers,
twelve were homomorphically fertilized by their own-form pollen, taken
in every case from a separate plant; and not one flower set a
seed-capsule: twelve other flowers were heteromorphically fertilized by
pollen from short-styled flowers; and they set nine pods, each
including on an average seven good seeds: as before, ten seeds is the
maximum possible production. Secondly, of the short-styled flowers,
twelve were homomorphically fertilized by own-form pollen, and they
yielded one capsule, including only three good seeds; twelve other
flowers were heteromorphically fertilized by pollen of long-styled
flowers, and these produced nine capsules, but one was bad; the eight
good capsules contained on an average exactly eight good seeds each.

The many flowers on the eleven long-styled plants under the net,
which were not fertilized, produced only three capsules (including 8,
4, and 1 good seeds); whether, owing to the interlocking of the
branches, these accidentally received pollen from the other form, I
will not pretend to conjecture. The single long-styled plant which was
uncovered, and grew close by the uncovered short-styled plant, produced
five good pods; but it was a very poor and small plant.

The flowers borne on the thirteen short-styled plants under the net,
which were not fertilized, produced twelve capsules (containing 5.6
seeds on average): as some of these capsules were very fine, and five
were borne on one twig, I suspect that they had been visited by some
minute insect which had accidentally got under the net and had carried
pollen from the other form. The one uncovered short-styled plant
yielded exactly the same number of capsules, namely, twelve.

From these facts we have some evidence, as in the case of L.
grandiflorum, that the short-styled plants are in a very slight
degree more fertile with their own pollen than are the long-styled
plants. And we have the clearest evidence, from the result of the
forty-eight flowers artificially fertilized, that the stigmas of each
form require pollen from the stamens of corresponding height produced
by the opposite form.

In contrast with the case of L. grandiflorum, it is a
singular fact that the pollen-grains of both forms of L. perenne when
placed on their own-form stigmas, though not causing fertility, yet
emit their tubes; and these tubes I found, after an interval of
eighteen

hours, had penetrated the stigmatic tissue, but to what depth I did
not ascertain. In this case the inaction of the pollen-grains on their
own stigmas must be due either to the tubes not reaching the ovules, or
reaching them and not efficiently acting on them. In the case of Lythrum
Salicaria,1 which I hope at some future time to lay
before the Society, there are three distinct forms, each of which
produces two kinds of pollen; but neither pollen, when placed on its
own stigma, causes fertility, except occasionally and in a very
moderate degree; yet the pollen-tubes in each case freely penetrate the
stigmatic tissue.

The plants of L. perenne and L. grandiflorum grew,
as stated, with their branches interlocked, and with scores of flowers
of the two forms close together; they were covered by an open net,
through which the wind, when high, passed; and such minute insects as Thrips
could not, of course, be excluded; yet we have seen that the
utmost possible amount of accidental fertilization on seventeen
long-styled plants in the one case, and on eleven plants in the other
case, was the production, in each, of three poor capsules; so that we
may infer that, when the proper insects are excluded, the wind does
hardly anything in the way of carrying pollen from plant to plant. I
allude to this fact because botanists, in speaking of the fertilization
of plants or of the production of hybrids, often refer to the wind or
to insects as if the alternative were indifferent. This view, according
to my experience, is entirely erroneous. When the wind is the agent in
carrying pollen, either from one separated sex to the other, or from
hermaphrodite to hermaphrodite (which latter case seems to be almost
equally important for the ultimate welfare of the species, though
occurring perhaps only at long intervals of time), we can recognize
structure as manifestly adapted to the action of the wind as to that of
insects when they are the carriers. We see adaptation to the wind in
the incoherence of the pollen, in the inordinate quantity produced (as
in the Coniferæ, Spinage, &c.), in the dangling anthers well
fitted to shake out the pollen, in the absence or small size of the
perianth or in the protrusion of the stigmas at the period of
fertilization, in the flowers being produced before they are hidden by
the leaves, in the stigmas being downy or plumose (as in the Gramineæ,
Docks, and other plants) so as to secure the chance-blown
grains. In plants which are fertilized by the wind, the flowers do not
secrete nectar, their pollen is too incoherent to be easily collected
by insects, they have not bright-coloured corollas to serve as guides,
and they are not, as far as I have seen, visited

by insects. When insects are the agents of fertilization (and this
is incomparably the more frequent case both with plants having
separated sexes and with hermaphrodites), the wind plays no part, but
we see an endless number of adaptations to ensure the safe transport of
the pollen by the living workers. We can recognize these adaptations
most easily in irregular flowers; but they do not the less occur in
perfectly regular flowers, of which those of Linum offer an
instance, as I will almost immediately endeavour to show.

I have already alluded to the rotation of each separate stigma in
the long-styled form alone of Linum perenne. In the other
species examined by me, and in both forms when the species are
dimorphic, the stigmatic surfaces face the centre of the flower, and
the furrowed backs of the stigmas, to which the styles are attached,
face the circumference. This is the case, in the bud, with the stigmas
of the long-styled flowers of L. perenne. But by the time
the flower in this form has expanded, the five stigmas, by the torsion
of that part of the style which lies beneath the stigma, twist round
and face the circumference. I should state that the five stigmas do not
always perfectly turn round, two or three often facing only obliquely
towards the circumference. My observations were made during October;
and it is not improbable that earlier in the season the torsion would
have been more perfect; for after two or three cold and wet days the
movement was very incomplete. The flowers should be examined shortly
after their expansion; for their duration is brief, and, as soon as
they begin to wither, the styles become spirally twisted together, and
the original position of the parts is lost.

He who will compare the structure of the whole flower in both forms
of L. perenne and grandiflorum, and, I may add, of
L. flavum, will, I think, entertain no doubt about the
meaning of this torsion of the styles in the one form alone of L.
perenne, as well as the meaning of the divergence of the stigmas
in the short-styled forms of all three species. It is absolutely
necessary, as we now know, that insects should reciprocally carry
pollen from the flowers of the one form to those of the other. Insects
are attracted by five drops of nectar, secreted exteriorly at the base
of the stamens, so that to reach these drops they must insert their
proboscides outside the ring of broad filaments, between them and the
petals. In the short-styled form of the above three species, the
stigmas face the axis of the flower; and had the styles retained their
original upright and central position, not only would the stigmas have
presented their backs to insects as they sucked the flowers, but they

would have been separated from them by the ring of broad filaments,
and could never have been fertilized. As it is, the styles diverge
greatly and pass out between the filaments. The stigmas, being short,
lie within the tube of the corolla; and their papillous faces, after
the
divergence of the styles, being turned upwards are necessarily brushed
by every entering insect, and thus receive the required pollen.

In the long-styled form of L. grandiflorum, the parallel
anthers and stigmas, slightly diverging from the axis of the flower,
project only a little above the tube of the somewhat concave corolla;
and they stand directly over the open space leading to the drops of
nectar. Consequently when insects visit the flowers of either form (for
the stamens in this species occupy the same position in both forms),
they will get their proboscides well dusted with the coherent pollen.
As soon as the insect inserts its proboscis to a little depth into the
flower of the long-styled form, it will necessarily leave pollen on the
faces and margins of the long stigmas; and as soon as the insect
inserts its proboscis to a rather greater depth into the short-styled
flowers, it will leave pollen on their upturned stigmatic surfaces.
Thus the stigmas of both forms will indifferently receive the pollen of
both forms; but we know that the pollen alone of the opposite form will
produce any effect and cause fertilization.

Long-styled form of L. perenne, var. Austriacum,
with the petals and calyx removed on the near side.

In the case of L. perenne, affairs are arranged a little
more perfectly; for the stamens in the two forms stand at different
heights,

and pollen will adhere to different parts of an insect's body, and
will generally be brushed off by the stigmas of corresponding height,
to which stigmas each kind of pollen is adapted. In this species, the
corolla is flatter, and in the one form the stigmas and in the other
form the anthers stand at some height above the mouth of the corolla*.
These longer stigmas and longer stamens do not diverge greatly; hence
insects, especially rather
small ones, will not insert their proboscides between the stigmas or
between the anthers, but will strike against them, at nearly right
angles, with the backs of their head or thorax. Now, in the long-styled
flowers of L. perenne, if each stigma had not rotated on its
axis, insects in visiting them would have struck their heads against
the backs of the stigmas; as it is, they strike against the papillous
fronts of the stigmas, and, their heads being already charged with the
proper coherent pollen from the stamens of corresponding height borne
by the flowers of the other form, fertilization is perfectly effected.

Thus we can understand the meaning of the torsion of the styles in
the long-styled flowers alone, as well as their divergence in the
short-styled flowers.

One other point is worth a passing notice. In botanical works many
flowers are said to be fertilized in the bud. This rests solely, as far
as I can discover, on the anthers opening in the bud; no evidence is
adduced that the stigma is at this period mature, or that, if then
penetrated by pollen-tubes, it is not subsequently, after the expansion
of the flower, acted on by pollen brought from other flowers. In the
case of Cephalanthera grandiflora I have shown† by
experiment that insufficient precocious self-fertilization, together
with subsequent full fertilization, is the regular course of events.
The belief that flowers of any plant are habitually fertilized in the
bud, or are perpetually self-fertilized, is a most effectual bar to
really understanding their structure. I am far from wishing to say that
some flowers, in certain seasons, are not fertilized in the bud: I have
reason to believe that some flowers are frequently fertilized without
expanding; but my observations lead me to disbelieve that this is ever
the invariable

* I neglected to get drawings made from fresh flowers
of the two forms. Mr. Fitch1 has made the above sketch of a long-styled
flower from dried specimens and published engravings: his well-known
skill ensures accuracy in the proportional size of the parts; and I
believe their relative position is true.

course with all the flowers of any species whatever. As it is
difficult to prove without troublesome experiments the falsity of the
belief of regular fertilization in the bud, I here notice this subject.
An estimable and laborious observer*, resting his belief on the usual
kind of evidence, states that in L. Austriacum (which is
dimorphic and is considered by Planchon1 as a variety of L. perenne)
the anthers open the evening before the expansion of the flowers, and
that the long-styled stigmas are then almost always fertilized. He asks
whether this precocious fertilization in the several species of Linum
and in other plants is not one cause of the short duration of
their flowers. Now we know positively that, so far from Linum
perenne being fertilized by its own pollen in the bud, its own
pollen is as powerless on the stigma as so much inorganic dust.

Linum flavum.—To recur to our more immediate subject, in
the long-styled form of this species the pistil is nearly twice as long
as in the short-styled form; and the stigmas are longer with the
papillæ coarser. In the short-styled form the stigmas diverge and pass
out between the filaments. The stamens in the two forms differ in
height, and, what is singular, the anthers of the longer stamens are
shorter; so that in the short-styled form both stigmas and anthers are
shorter than in the other form. The pollen of the two forms does not
differ. I have not been able to try any experiments on this species;
but a careful observer, Mr. W. C. Crocker, intends proving their
reciprocal fertility next summer. As this plant is propagated by
cuttings, I have generally found that all the plants in the same garden
belong to the same form. On inquiry I have never heard of its seeding
in this country; but to anyone wishing to raise seedlings, in all
probability the path is now open, namely, by carrying pollen from one
form to the other.

I have now shown that three species of Linum are
dimorphic, besides several races of L. perenne, esteemed by
some botanists to be distinct species, such as L. montanum, L.
Sibiricum, and L. Austriacum. According to Vaucher†2, L.
Gallicum, L. maritimum, and L. strictum are
in the same
manner dimorphic, as likewise is, according to Planchon‡, L.
salsoloides. This latter botanist is the only one who seems to
have been struck with the importance of the subject; and he acutely
asks whether this dimorphism has not some influence on the manner of
fertilization. We thus know of

seven dimorphic species of Linum;but as this
structure has been overlooked in such common garden-flowers as L.
grandiflorum
and L. flavum, it is probably of frequent occurrence.

All the species, however, are certainly not thus characterized. I
have examined many specimens of L. catharticum, and found in
all that the stamens and stigmas were of nearly equal height and the
same in all the plants. So, again, I looked, near Torquay, at many
flowers of the wild L. usitatissimum or angustifolium (I
know not which), and there was no trace of dimorphism. Again, I raised
111 plants from seed sent me from Kew, incorrectly named L.
Austriacum; the plants were tall and straight, having a rather
different aspect from the wild species seen at Torquay, with extremely
fugacious blue flowers: in all these plants the stigmas stood on a
level with the anthers or projected a very little above them. I
protected the flowers from insects; but every one of the 111 plants
produced plenty of seed. I mention this fact because it had occurred to
me that possibly a species might be dimorphic in function, though not
in structure.

Lastly, Linum Lewisii, which is ranked by Planchon as a
variety of L. perenne, but which, now that we know the
meaning of reciprocal dimorphism, surely deserves specific honours,
must not be passed over. According to Planchon*, the same plant bears
some flowers with anthers and stigmas of the same height, and others
with styles either longer or shorter than the stamens; so that the same
individual plant is
trimorphic. This, as far as I know, is a unique case. From analogy we
may pretty safely predict the function of the three kinds of flowers:
those with stigmas and anthers of the same height will be self-fertile;
those with these organs of unequal height will require reciprocal
fertilization. A plant of L. grandiflorum or of the other
dimorphic species, growing by itself, could no more perpetuate its race
than could one sex of a diœcious plant, nor could any number of plants
without the aid of insects. A single plant of Linum Lewisii,
on the other hand, in all probability could propagate itself, even if
no insects were present, as probably sometimes occurs in its Arctic
home. If insects visited the plant, the flowers which were dimorphic
would be fertile one with another or with those on any neighbouring
plant. Thus the plant would receive the advantage of a cross.

* Hooker's London Journ. of Botany, 1848, vol. vii.
p. 175. It is not improbable that the allied genus Hugonia is
dimorphic; for (p. 525) one species is described "staminibus exsertis;"
another has "stamina 5, majora, stylos longe superantia;" and another
is furnished "stylis staminibus longioribus."

That this is an advantage, and is one great end gained by reciprocal
dimorphism, I can entertain no doubt. That in some cases this
dimorphism may be a step towards a complete separation of the sexes, I
will not dispute; but good reasons could be assigned to show that there
is no necessary connexion between reciprocal dimorphism and a tendency
to diœcious structure. Although good is gained by the inevitable
crossing of the dimorphic flowers, yet numerous other analogous facts
lead me to conclude that some other quite unknown law of nature is here
dimly indicated to us.